Old vehicle brake systems are mostly hydraulic ones which use hydraulic power as their driving source. However, with the introduction of sophisticated brake controls such as anti-lock brake systems (ABS's), electric brake systems are gathering much attention these days because electric brake systems are capable of improving the performance of these brake controls. Such an electric brake system includes an electric motor as a driving source for a braking mechanism for applying a braking force to each vehicle wheel, and configured to generate the braking force by actuating the motor when the brake pedal is operated.
The applicant of this application has proposed an electric brake system having the parking brake function, namely the function of continuously applying a parking brake to a wheel of a vehicle while the vehicle is parked (below-identified Patent document 1).
The electric brake system with a parking function disclosed in Patent document 1 includes an electric motor, a braking mechanism configured to press a friction pad against a rotor which rotates together with a wheel of a vehicle, under the power of the electric motor, thereby applying a braking force to the wheel, an intermediate gear disposed in the power transmission path through which the power of the electric motor is transmitted to the braking mechanism, and a locking pin capable of engaging in and disengaging from any one of engaging holes formed in a side surface of the intermediate gear. The locking pin can be advanced and retracted between a parking/locking position at which the locking pin is engaged in one of the engaging holes, and an unlocking position at which the locking pin is engaged in none of the engaging holes. By advancing the locking pin with a locking pin-driving locking actuator, the locking pin can be engaged in one of the engaging holes, thereby preventing rotation of the intermediate gear.
To apply the parking brake, the electric motor is actuated to apply the braking force, and with the braking force applied to the wheel, the locking pin is advanced by the locking actuator until the locking pin is engaged in one of the engaging holes formed in the side surface of the intermediate gear.
When the parking brake operation has completed, torque in the brake-releasing direction (reaction force torque) is being applied to the intermediate gear due to the reaction force to the pressing force with which the friction pad is pressed against the disk rotor, so that the locking pin is kept engaged in the engaging hole by the frictional resistance generated between the intermediate gear and the locking pin due to the reaction force torque. This allows the braking force to be continuously applied to the vehicle wheel even after the electric motor is de-activated, while the vehicle is parked.